Recently, there has been rapid progress in weight reduction, thinning and miniaturization of electronic devices, and there is the requirement that electronic components that are to be amounted be reduced in weight, thinned, and miniaturized.
At present, for many electronic devices, solid solder is used as a material for connecting an electronic component and a wiring substrate. To mount the electronic component on the wiring substrate, solder paste is printed on an electrode pad in which a nickel or gold thin film has been formed. The solder is heated to melt at a temperature equal to or higher than the melting temperature of the solder, and the electrode pad and the solder are connected. To mount the electronic component on the wiring substrate, the electrode pad of the electronic component and the electrode pad of the wiring substrate are aligned with each other under a normal temperature. Then, the temperature is increased until spare solder melts on the electronic component and the wiring substrate. After the solder has been wetted to spread on the electronic component and the wiring substrate, the temperature is decreased. Thus, by decreasing the temperature of the electronic component and the wiring substrate to solidify the solder, a solid connection portion is formed between the electronic component and the wiring substrate.
As the package structure of the electronic component, there is a structure referred to as a QFP (Quad Flat Package). The QFP is a structure where an electric signal is transferred between the electronic component and the wiring substrate by locating the connection terminal of the electronic component on the outer periphery of the package and connecting the connection terminal to the wiring substrate.
However, in the QFP, an input-output terminal can be disposed only on the outer periphery of the package. Consequently, a dead space having no connection terminal is generated near the package center. This makes the QFP unsuitable as a structure for accommodating any increase in input-output terminals that occurs when there is an increase in the amount of information, and the QFP is disadvantageous in that it does not have high density capability.
To solve such a problem, a structure referred to as a BGA (Ball Grid Array) having external connection terminals in both the outer periphery and the center of the electronic component is used. The BGA structure is a structure where high density mounting can be achieved by forming an outer connection terminal on the entire surface of a semiconductor element. However, the following problems occur even in such a structure where high density mounting can be achieved.
Generally, the linear expansion coefficient of the electronic component and the linear expansion coefficient of the wiring substrate are different from each other. Due to heating and cooling during operation of the electronic component, a difference is generated in the linear expansion coefficient between the electronic component and the wiring substrate. The difference in linear expansion coefficient generates stress on solid solder that is used to connect the electronic component and the wiring substrate, thus causing a creeping phenomenon which is seen in a solid metal material. Distortion which is generated by the creeping phenomenon accumulates in the connection portion between the wiring substrate and the electronic component to cause cracks in the connection portion between the wiring substrate and the electronic component. The cracks, when they grow, lead to disconnection of the connection portion between the wiring substrate and the electronic component.
The recent computerization of an automobile functions has been accompanied by cases in which electronic devices, such as devices within the engine housing, or the like, operate in a high temperature environment of 130° C. or higher. In such an environment, a creeping phenomenon, in particular, easily occurs. Thus, an electronic device capable of maintaining highly accurate connection reliability even under such a high temperature environment is required.
A technology for the electronic device capable of maintaining highly accurate connection reliability is described in, for example, JP11-163049A (Patent Literature 1), JP2003-020404A (Patent Literature 2), and JP4-240741A (Patent Literature 3).
Patent Literature 1 describes a method for reinforcing a solder connection portion by supplying and curing a liquid resin referred to as underfill resin in the solder connection portion between the electronic component and the wiring substrate. According to this method, stress which is generated between the electronic component and the wiring substrate is dispersed not only to the solder connection portion but also to the entire surface of the underfill resin.
Patent Literature 2 describes a technology for reducing creeping distortion by forming a flexible stress relaxation layer in a connection portion between the electronic component and the wiring substrate.
Patent Literature 3 describes a structure for applying a low melting-point metal in a connection portion. More specifically, a structure is described, where a gold bump is used as the bump of a semiconductor side, the low melting-point metal is applied to its leading end, and the semiconductor element and the wiring substrate are connected. Thus, stress within a temperature condition range where the low melting-point metal melts is relaxed, and reliability improvement can be expected.